AAC Bit

Note that for a potentially dynamic ECU that does not occur just once in the network, the configuration under “Self-configurable”, as described in the previous chapter Solution and Configurations, must be supported. An ECU with this configuration can be recognized by the set MSB “Arbitrary Address Capable” in the NAME. Reason: this bit is set only for ECUs that can search for new addresses independently. If an ECU occurs exclusively (just once) in the network, a fixed device address can be used. The “Arbitrary Address Capable” bit then does not have to be set. The logical result of this is that all self-configurable ECUs have a NAME with lower priority than all ECUs for which this bit is not set.

Request

An important mechanism in the J1939 network management is the use of the PGN ‘Request’. This PGN can be used, in turn, to request any PGNs from ECUs. Thus, it can also be applied to the ‘Address Claimed’ PGN. The ECU is obligated to answer this “Request for Address Claimed” with the ‘Address Claimed’ PGN.

Important: If the requesting ECU has a valid address, it must also answer its own request!

NULL Address

The request mechanism is frequently applied in dynamic networks and used at the beginning of communication. Consequently, an ECU that is to participate in communication can request the address situation before the “Address Claim”. Note: The ‘Request for Address Claimed’ PGN represents a major exception in J1939. Namely, it is the only parameter group that may be sent without a valid address, thus from the NULL address (254).